1. Field of the Invention
The present invention relates to a cleaning method and a cleaning apparatus of a semiconductor surface such as silicon surface, in which a residual water stain, the so-called water mark can be reduced in a step of drying silicon after wet cleaning or etching of a silicon surface.
2. Description of the Related Art
In the semiconductor industry using silicon, the material of silicon is classified into single crystal silicon, polycrystalline silicon, and amorphous silicon in view of the configuration. Silicon in any configuration has characteristics as a semiconductor, and plenty of raw material exists on earth. Thus, silicon is used in a wide field. The single crystal silicon is used for a memory such as a DRAM and EPROM or an arithmetic unit such as a CPU and MPU. The polycrystalline silicon is used for a switching transistor and a driving circuit of a liquid crystal display device, or a solar cell. The amorphous silicon is used for a switching transistor of a liquid crystal display device or a solar cell.
Among applications of each silicon, silicon is often used particularly for an integrated circuit using the function as a transistor. Especially, in the case where silicon is used for an integrated circuit, research and development competition are keen to make a clean environment or a micro machining apparatus and to develop material techniques, for the sensitivity of silicon to impurities and minute machining technique.
The technique of cleaning and etching is indispensable when silicon is used as a semiconductor. Cleaning is carried out in a wet mode in most cases. As the cleaning for removing physically adsorbed materials, there are scrub cleaning in which cleaning is carried out by scrubbing the surface by a brush or the like, ultrasonic cleaning or megasonic cleaning in which cleaning is carried out by the impact of compressed waves generated in pure water or a solution by ultrasonic waves, or the like. As the cleaning for removing organic materials attached to the surface, there is cleaning in which silicon is immersed in oxygenated water (hydrogen peroxide water) mixed with sulfuric acid to remove the organic materials by a chemical reaction, or the like. As the cleaning for removing metal contaminants, there is cleaning in which silicon is immersed in oxygenated water mixed with hydrochloric acid to remove the metal contaminants by a chemical reaction, or the like.
As to the etching, there are dry etching using a gas and wet etching using a solution. The wet etching using a solution is used for etching of the entire of a wide surface or for a case where a processed dimension is relatively large in semiconductor micro working.
As described above, in the semiconductor techniques, a wet process is often used in cleaning, etching and the like in the present circumstances. In this wet process, a water mark is a serious problem. The water mark is such a phenomenon that when a substrate having a silicon surface is dried after wet cleaning or etching, a waterdrop adheres to the substrate surface during the period in which the state of the substrate is changed from a wet state to a dry state, and although the adherent waterdrop evaporate by drying, a mark of the waterdrop remains after the waterdrop disappears.
A suitable Japanese word for the water mark is not established for skilled persons in the art. Various expressions such as a water trace, waterdrop trace, or stain of water are used, and in the present specification, we uses the term of water mark. A definite technical interpretation is not established for what the water mark is, and there is no interpretation beyond the level of a hypothesis.
The following three elements are indispensable for the phenomenon in which the water mark is formed. They are (1) silicon, (2) oxygen, and (3) water. If even one of them is lacking, the water mark is not formed. FIG. 3 shows a commonly accepted theory (for example, Semiconductor World (monthly) 1996. 3, pages 92-94). In step a), oxygen in a dry atmosphere is dissolved in a waterdrop (H2O pure water) adherent to the surface of silicon. In step b), the dissolved oxygen is diffused into the interface between the silicon surface and the waterdrop. In step c), an oxide is formed on the silicon surface. In step d), the formed oxide dissolves into silicic acid (presumed to be H2SiO3). In step e), silicic acid is diffused in the liquid and is dissociated, and then it is further diffused. After the waterdrop is dried, the silicon oxide remains on the silicon surface, and this is regarded as the water mark.
If the water mark is once formed, it is extremely difficult to remove it, and substantially it is impossible to remove it. Thus, the water mark functions as a mask at subsequent etching of silicon, so that etching can not be made because of the water mark for a portion where etching is desired, or only partial etching can be made. As a result, silicon at the portion remains unetched.
The size and the number of water marks are greatly changed by conditions at the formation. The size is about 1 xcexcmxcfx86 to 60 xcexcmxcfx86, and the number is from several to one thousand or more on a substrate of 5 inches xcfx86 or 5 inches square. In some case, several water marks of several hundred xcexcmxcfx86 are formed.
Since it is difficult to remove the water mark when it is once formed, it is important how to prevent the formation. Measures for preventing the formation of the water mark are summarized in the following two measures:
(1) To make the three elements of silicon, oxygen, and water become incomplete.
(2) Not to give a time for reaction (to make a time from water washing to drying as short as possible).
The measure (2) means instantaneous drying, and actually, there is a spin drying method in which a substrate is rotated to dry the substrate by using an air flow and centrifugal force. However, in this method, it is impossible to dry the substrate for such a short time that the water mark is not formed. Thus, the spin drying method is unsuitable for drying in the state where the three elements in the above measure (1) are complete.
As a method of realizing quick drying and eliminating water, there is an IPA (also called isopropyl alcohol, propyl alcohol, or propanol) vapor drying method. In this method, the IPA is heated to produce a vapor. When a substrate is placed in the IPA vapor filling a vessel in a drying apparatus, the vapor of IPA condenses on the substrate, and is replaced with water on the substrate in a short time. According to this IPA vapor drying method, since water and IPA are replaced with each other in a short time, water can be removed from the three elements of the measure (1), and short time drying of the measure (2) can be achieved at the same time. Thus, it is possible to prevent the formation of a water mark at a considerably high probability. Thus, the method is applied to almost all cases in current steps of using a silicon semiconductor.
As other recent methods, a drying system such as a Marangoni system or IPA direct substitution system is proposed, and such a system is actually used in some cases. In the Marangoni system, a substrate is pulled up slowly from pure water into the atmosphere of IPA and nitrogen, and the surface tension of pure water at that time is used. Similarly to the IPA direct substitution system, it can make the water mark zero in principle.
As a method of removing oxygen from the three elements for the formation of a water mark, there are proposed a method of drying a substrate in a nitrogen atmosphere in a closed system, a low pressure drying method of drying a substrate in a low pressure state, and the like. However, it is difficult to completely remove oxygen even in a carrier system in which a treatment is carried out in a state where a substrate is put in a normal carrier, or even in a closed space in a carrierless system (or single wafer processing type) in which only a substrate is processed without enclosing the substrate in a carrier, and it is also impossible to replace oxygen with other gas such as nitrogen in a short time. Also in the low pressure drying method, water and oxygen exist until a low pressure is achieved, so that it is very difficult to prevent the occurrence of a water mark also in this method.
The above described conventional methods of preventing the occurrence of a water mark are such that oxygen or water is removed from the three elements of silicon, oxygen and water for the occurrence of a water mark, or drying is carried out as quickly as possible. There is also a method of removing silicon from the three elements. When a natural oxide film exists on a silicon surface, the water mark is not formed (see the foregoing Semiconductor World 1996. 3, pages 92-94). However, in this method, it is important how to control a natural oxide film to make thin. An uncontrolled thick natural oxide film causes deterioration of a device. Moreover, the method can not be used in a step in which a trace of natural oxide film is not permitted.
As a method of forming a thin oxide film, there is known a chemical oxidation film producing method disclosed in xe2x80x9cUltra Clean ULSI Technique, Tadahiro Ohmi, Baifukan Co., page 214xe2x80x9d. This chemical oxidation film producing method is applied prior to a step of forming a gate oxide film on a silicon surface, which is especially important when an insulated gate field effect transistor is formed on silicon. In the formation of the gate insulating film, silicon oxide is formed on the silicon surface by using a thermal oxidizing method based on a dry oxidizing technique. Since a channel for flowing carriers is formed in the interface between the silicon oxide and silicon, an oxide film is formed to prevent the silicon surface from being polluted during the transfer to an oxidizing apparatus after cleaning the silicon surface.
Silicon on which the chemical oxidation film was formed is transferred to a thermal oxidizing step as it is, and a thermal oxidation film is formed on the chemical oxidation film. The chemical oxidation film becomes a part of the gate oxide film as it is. Accordingly, the step of forming the chemical oxidation film becomes complicated.
Table 1 shows the manufacturing procedure for forming the chemical oxidation film. The chemical oxidation film is formed through 5 wet steps and 1 dry step, that is, through 6 steps in total.
When the chemical oxidation film is formed on silicon after a cleaning step, it is possible to prevent the occurrence of a water mark. Since the chemical oxidation film is a precisely controlled oxide film, the film thickness is also controlled and few impurities are also included in the chemical oxidation film, unlike a natural oxide film.
However, when the step of forming this chemical oxidation film is introduced into all of the normal cleaning steps of silicon, the number of steps is greatly increased.
The present inventors paid attention to silicon among the three elements which should not be made complete in order to prevent the occurrence of a water mark. Since the steps of the method using a chemical oxidation film are complicated, the inventors considered forming a controlled oxide film by a simpler means. In general, a substrate is cleaned or etched, and is rinsed by pure water, and then drying is carried out. The inventors have found that a controlled thin oxide film can be formed on a silicon surface during the rinse or after the rinse before drying, and the present invention has been achieved.
An object of the present invention is therefore to provide a cleaning method in which in order to prevent the occurrence of a water mark, in a method of removing silicon from the three elements (silicon, oxygen, and water) for the occurrence of a water mark, without using complicated steps such as a step of forming a chemical oxidation film, and without using an uncontrolled natural oxide film, on the contrary, the natural oxide film is removed in view of subsequent steps and device characteristics, and on the silicon surface with no natural oxide film, a controlled oxide film is formed in pure water by a simple method during a pure water rinse or after the rinse after cleaning or etching, whereby the occurrence of a water mark at drying is prevented.
In order to achieve the above object, according to an aspect of the present invention, a cleaning method of silicon is characterized in that after a substrate having a silicon surface is wet cleaned, the silicon surface is rinsed by pure water, an oxide film with a thickness of 10 to 30 xc3x85 is formed on the silicon surface during the rinse or after the rinse before drying, and the silicon surface is dried.
According to another aspect of the present invention, a cleaning method of silicon is characterized in that after a substrate of silicon with a surface of a natural oxide film is subjected to wet etching to remove the natural oxide film, the silicon surface is rinsed by pure water, and an oxide film with a thickness of 10 to 30 xc3x85 is formed on the silicon surface during the rinse or after the rinse before drying, and then the silicon surface is dried.
According to still another aspect of the present invention, in a cleaning method of a substrate before a step in which amorphous silicon is irradiated with laser light to make polycrystalline silicon, a cleaning method of silicon is characterized in that after the substrate including amorphous silicon with a surface of a natural oxide film is subjected to wet etching to remove the natural oxide film, the surface of amorphous silicon is rinsed by pure water, an oxide film with a thickness of 10 to 30 xc3x85 is formed on the amorphous silicon surface during the rinse or after the rinse before drying, and the amorphous silicon surface is dried.
In the above cleaning method, it is preferable to form the oxide film with a thickness of 10 to 30 xc3x85 in a wet manner using pure water added with an oxidizer. It is preferable that the pure water added with the oxidizer is produced by such as adding ozone obtained by electrolysis of pure water into pure water, adding ozone obtained by ultraviolet irradiation to oxygen into pure water, or adding hydrogen peroxide of 5 to 20 vol % into pure water.
According to still another aspect of the present invention, a wet type silicon cleaning or etching apparatus comprises means for cleaning or etching silicon, means for rinsing a silicon surface by pure water, means for supplying pure water added with an oxidizer to the silicon surface, and means for spin drying the silicon surface, and is characterized in that before the silicon surface is dried by the means for spin drying, the pure water added with the oxidizer is supplied from the means for supplying the pure water.
In the foregoing silicon cleaning apparatus, it is preferable that the pure water added with the oxidizer is produced by such as adding ozone obtained by electrolysis of pure water into pure water, adding ozone obtained by ultraviolet irradiation to oxygen into pure water, or adding hydrogen peroxide of 5 to 20 vol % into pure water.
The present invention provides a cleaning method for preventing the occurrence of a water mark at drying and provides a cleaning apparatus for realizing the cleaning method, and is characterized in that in order to prevent the occurrence of a water mark, in a method of removing silicon from the three elements (silicon, oxygen, and water) for the occurrence of a water mark, on a silicon surface on which a natural oxide film does not exist, a controlled oxide film is formed in pure water by a simple method during or after a pure water rinse after cleaning or etching.
After wet cleaning or wet etching of silicon, a rinse by pure water is carried out to wash a cleaning liquid or etchant off from the silicon surface. During this rinse, pure water exists over the entire of silicon and the pure water flows, so that a water mark is not formed. That is, if a controlled oxide film is formed while silicon exists in the pure water after the achievement of the object of cleaning or etching, the occurrence of a water mark can be prevented.
By means of adding an oxidizer into pure water, a controlled oxide film as an object of the present invention is formed in pure water. At the cleaning of silicon, according to its object, there are cleaning for removing physical adsorbed materials, cleaning for removing adherent organic materials, cleaning for removing metal contamination, and the like. In any cleaning, a treatment solution for cleaning is used. In a wet etching step of silicon, an etchant is used as a treatment solution. In order to wash the cleaning solution or etchant off from the silicon surface, a rinse by pure water is carried out.
In the step of rinse by pure water, the type of rinse, time of rinse, and the quantity of pure water are different between a wet cleaning or etching step carried out for a substrate together with a carrier into which the substrate is put and a single wafer processing type in which only substrate is rinsed. However, there is a step for flowing pure water to wash off the solution remaining on the substrate surface for some constant time. In this rinse step, after the solution on the substrate surface is sufficiently washed off, an oxidizer is added into the pure water for rinse, and an oxide film is formed on the silicon surface. Alternatively, pure water for rinse is changed to pure water added with an oxidizer.
In the present invention, basically, instead of pure water used in a pure water rinse, pure water added with an oxidizer is flown similarly to the rinse of the substrate, so that an oxide film is formed. There was no concept before the present invention that in a cleaning step, drying is carried out after an oxide film is formed on a cleaned silicon surface.
As an oxidizer, as a result of various experiments through trial and error, the present inventors have reached the conclusion that an effect is obtained when an oxidizer obtained by adding ozone into pure water or by adding hydrogen peroxide into pure water is used. As a method of producing ozone, the following methods were tried: a method of producing ozone by electrolysis of pure water; a method of producing ozone by irradiation of ultraviolet rays to oxygen before bubbling in a system in which pure water is bubbled by oxygen; in the case of a single wafer processing type rinse, a method of producing ozone by irradiation of ultraviolet rays to pure water. As a result, it was found that any of the methods are effective.